1. Field of the Invention
The present invention generally relates to growth of InGaN-based structures. More particularly, but not by way of limitation, the present invention relates to the growth of high-quality InGaN-based heterostructures that can be suitable for use in high-efficiency optoelectronic devices such as, for example, photovoltaic cells.
2. Description of Related Art
Worldwide energy demand is growing at high speed with the rapid economic development of many nations. As one of the forefront technologies for clean, renewable energy, there is much demand for new solar cell technologies. The use of photovoltaic devices that can absorb and convert light into electrical power has been limited by conversion efficiencies and high production costs. Even the fabrication of the simplest semiconductor cell is a complex process that has to take place under exactly controlled conditions, such as high vacuum and temperatures between 400 and 1,400 degrees Celsius. Current silicon based solar cells are inefficient and relatively expensive. GaAs-based solar cells for use in concentrator and space systems can be highly efficient, yet more expensive. The progression of efficiency of III-V solar cells has been more recent than for silicon, and is best illustrated by starting with a basic p-n junction device and adding materials layers and discussion as needed. A band diagram schematic of a basic p-n junction device is given in FIG. 1. As a simple p-n doped homojunction device, GaAs is hampered by a high absorption coefficient. High absorption within a diffusion length of the surface leads to large surface recombination losses and devices had low efficiencies (˜10%) [2].
A breakthrough in the early 1970's led to the formation of a heteroface or buried homojunction device with a high bandgap AlGaAs “window” at the incident light surface. The bandgap of AlAs is 2.15 eV and for GaAs is 1.43 eV, with alloy bandgap energies ranging between the two. Light incident on such a device will first encounter the high bandgap window where high energy photons will be absorbed and mostly lost due to surface recombination. However, many of the photons will be transmitted further into the device and absorbed away from the surface. Devices utilizing this window had greatly improved efficiency (16%, AM1, sea level), indicating that the effective diffusion length of carriers, considering both the bulk and surface recombination, is greater with the window material [28].
Interface defects are one problem that occurs with the window approach due to lattice mismatch between the AlGaAs window and GaAs cell. The lattice parameters of GaAs and AlAs are 5.653 Å and 5.660 Å respectively, leading to a lattice mismatch of only ˜0.15%. Even though alloy mismatch is very small, interface states are formed and lead to recombination loss. More-recent cells have improved on this problem by using different III-V materials [11, 21, 30]. The switch of materials is also partly due to the problem of oxygen related defects present in AlGaAs [30]. FIG. 2 shows one example of a recent triple junction solar cell. While recent cells are extremely complicated, they continue to use the same window principle to reduce recombination losses. In this case, the top cell uses a lattice matched AlInP window to the InGaP junction, the middle cell uses an InGaP window to the InGaAs junction and the bottom cell uses an InGaAs buffer layer to help lattice match to the Ge junction/substrate [30]. Other materials possibilities are also utilized in other cells. Efficiency increases by use of a large bandgap “window” material to force absorption away from the surface, and by ensuring good lattice matching at all interfaces to avoid interface states or other recombination defects.
As discussed above, buried homojunction devices utilizing an AlGaAs window and single GaAs junction were first explored. Because GaAs has a direct bandgap of 1.43 eV, devices composed of GaAs are operable at relatively high temperatures. The temperature at which a device becomes inoperable generally depends on the material bandgap, doping and the temperature. At high temperatures the intrinsic carrier concentration becomes equal to the doping level and “kills” the device. GaAs cells often work well in concentrator systems where heat is a natural product of the solar concentration. However, as was shown for the case of a general silicon cell, the efficiency of a single material solar cell may be inherently limited. Single homojunction GaAs cells generally have maximum achievable efficiencies under concentration of ˜27-30% [2]. Because of this inherent material limitation, recent work has involved increasing efficiency by forming multi junction cells with two, three and even four junctions [11, 13, 21, 30].
FIG. 2 shows a recent high efficiency triple junction cell (33.3% efficiency, no concentration) [30]. For a given semiconductor, photons with energy below the bandgap are transmitted and lost. Similarly, high energy photons are absorbed, but the energy in excess of the bandgap is lost as carriers thermalize to the band edge. The highest efficiencies are achieved when the photon energy is closely matched to the bandgap. Multi junction devices use a larger bandgap junction at the surface to absorb high energy photons while transmitting lower energy photons to the next cell with smaller bandgap. Although some of the light may be lost as more junctions are added, this approach leads to record high efficiencies. The current efficiency record is 37.9%±2.3% and is held by a triple-junction GaInP/GaAs/GaInAs 2-terminal solar cell under 10 times concentration [10]. The efficiency of the device in FIG. 2 could be increased further by increasing the bandgap of the top cell from 1.86 eV to 1.96 eV by using AlInGaP [30]. Recent III-V devices are highly complex and sensitive to modifications—slight modifications to material parameters, compositions or even cell design can drastically affect cell performance.
Also related to the number of junctions, a recent advance involved the use of Ge as the substrate in place of GaAs, which conveniently also acts as an additional solar junction. Ge is an indirect bandgap semiconductor with a gap of 0.65 eV and a lattice parameter very close to GaAs and other common III-V alloys. Due to the small bandgap, cells utilizing a Ge substrate are no longer appropriate for high temperature operation. As a substrate, Ge is generally less expensive and tougher than GaAs, and, with proper doping, has the benefit of adding an extra working junction. Despite obvious benefits, the small lattice mismatch between Ge and GaAs of only 0.08% is enough to form interface misfit dislocations with a telltale crosshatch pattern. Because Ge acts as the substrate, it was necessary to modify the lattice parameters of all of the subsequent layers. The misfit dislocations were greatly decreased, and cell performance increased, by the introduction of about 1% indium into the InGaP and GaAs junction layers [30]. This example illustrates that lattice matching is critical to reduce defects and increase efficiency.
Another breakthrough that occurred in the 1970's was the use of highly doped tunnel junctions between each cell of a multijunction device. This technology has been refined, and is used in all high-efficiency multijunction cells today. The first tunnel junctions used in solar cells were generally thin, highly doped p-n junctions. The tunnel junction improves the efficiency of the device and should have low impedance to current flow, small potential drop across the junction and should transmit all of the light to the next cell in line [1]. Efficiency can be improved by reducing the tunnel junction thickness for less absorption and by making the bandgap larger than the surrounding cell so that more light is transmitted [11]. Recent tunnel junctions utilize double hetero-structures of p-AlInGaP/p-AlGaAs/n-(Al)InGaP/n-AlInGaP to increase the incident light onto the middle cell and to also produce potential barriers for minority carriers in the top and middle cell [3, 4]. Properly designed and implemented tunnel junctions lead to larger Jsc and φoc and greater efficiencies. Examples of nitride based tunnel junctions structures can be found in [17], [22], and [24]